1. Field of the Disclosure
The disclosure relates generally to power monitoring techniques for optical transmitters and, more particularly, to power monitoring systems used in feedback control of parallel optical transmitters.
2. Brief Description of Related Technology
Communication systems are called upon to transmit large volumes of data between users. Increasingly, these communication systems transmit such data through techniques that allow for simultaneous communication between users. In wireless communication systems, for example, simultaneous (or near simultaneous) communication can be achieved through channelizing data before broadcast—in essence segmenting an allotted bandwidth spectrum into different channel frequencies each assigned to different users for data communication. In optical communication systems, where optical fibers form at least part of the data transmission path, various techniques have been developed for simultaneous communication. Multimode fibers capable of simultaneously sustaining multiple light beams have been employed for short distance data communications. Such devices have been implemented using arrayed waveguide grating switches, for example. Increasingly, however, system designers have turned to parallel optical transmitters for more robust applications, over longer distances and when there is a premium on avoiding crosstalk and other errors.
Parallel optical transmitters typically rely upon a bank (or array) of lasers each capable of communicating individually through a bank (or array) of dedicated optical fibers. Recently, designers have turned to less complex architectures that avoid active or passive optical switching techniques common in the area. Instead dedicated, pre-assigned optical pairings between laser source and optical fiber are used. Often these parallel optical transmitters use a vertical cavity surface emitting laser (VCSEL) as the laser source.
In traditional systems, VCSEL lasers require constant or near constant monitoring of power levels, and in some cases in the monitoring of output channel wavelength. VCSELs, as well as other laser sources, are susceptible to degradation of performance over time (e.g., a reduction in output power). VCSELs can be susceptible to environmental conditions, such as temperature, which can alter the output power levels. While VCSELs are often batch fabricated, there can be fluctuations in the output power levels across the different VCSELs within the same parallel array device. In these instances, the same optical transmitter could have some data channels that transmit at higher, native power levels than other data channels.
Power monitoring thus is integral to ensuring acceptable operation of VCSEL-based transmitters. Typical power monitoring control is performed by measuring an output of a light emitting device and using this measurement to control the power supplied to the light emitting device. While some laser sources can be designed to emit a monitoring beam separately from the main data beam, because VCSELs typically only emit light from one surface, any monitoring must be from the same output beam used for data communications.
VCSELs are much cheaper and their surface emissions make them easier to integrate with other optical devices than the edge emitting lasers, so the use of VCSELs is very desirable.
Current attempts to monitor the power of VCSELS involve splitting off a portion of the output beam and, in some examples, applying that portion to a diffractive optical element which then splits the beam into two portions. These techniques are transmission-based in design—light must pass through the diffractive element to be split—and thus are useful in setting up power monitoring at both the laser transmitter side as well as at the fiber coupling side. The techniques, as well as other power monitoring techniques can be costly and overly difficult to manufacture. In the case of diffractive elements, these have to be imprinted, written, or otherwise formed on a substrate. Proper alignment is manufacturing intensive, and a different element must be used for each laser source. Other attempts at creating a monitor beam include physically forming a y-branch inside the main transmission substrate. These configurations, as with diffractive optical elements, require exact alignment to affect acceptable operation. Also, in either case, the close proximity desired between laser sources produces space constraints which also limit the effectiveness of conventional techniques.
The inventor has found there is a need for techniques that allow laser sources within a parallel optical transmitter to be monitored simultaneously to ensure proper device operation.